Study of defect-induced magnetic anisotropy in MWCNT and RGO dispersed SnTe using spin resonance and magnetic measurement

Abstract

Magnetism in topological materials has drawn significant attention due to its crucial role in stabilizing exotic quantum states. In topological crystalline insulators (TCIs), such as SnTe, the topological protection of energy bands arises from mirror symmetries in the crystal structure. When these symmetries are broken, new quantum phases, such as the quantum anomalous Hall and axion insulator states, can emerge. SnTe, a prototypical TCI, typically exhibits high bulk conductivity due to a large number of intrinsic Sn vacancies (10²¹ cm⁻³), which act as sources of free charge carriers. However, when SnTe nanograins are anchored onto multi-walled carbon nanotubes (MWCNTs) or reduced graphene oxide (RGO), the formation energy of Sn vacancies increases. This leads to a reduction in vacancy concentration, generation of internal stress, band gap widening, and an enhancement of weak antilocalization behavior. These effects are observed in the resulting composites MWCNT@SnTe (CTS) and RGO–MWCNT@SnTe (SRC), which show improved surface state conductivity. This enhancement is attributed to suppressed electron–phonon scattering and hole localization via charge transfer mechanisms. Moreover, these composites exhibit anisotropic behavior in surface and bulk carrier transport with respect to temperature and applied magnetic field. This anisotropy arises from the strain-induced band gap modification, which in turn reflects underlying magnetic anisotropy. The magnetic properties of CTS and SRC are further influenced by spin–orbit coupling (SOC), which induces spin canting charge transfer between components and temperature-dependent localization of electrons and holes. These combined effects contribute to the formation of bound magnetic polarons (BMPs) or spin clusters. At low temperatures and magnetic fields, CTS and SRC exhibit competing antiferromagnetic and ferromagnetic interactions, mediated by itinerant carriers through the Ruderman–Kittel–Kasuya–Yosida (RKKY) mechanism, which is activated by strain and band gap effects. At higher temperatures and magnetic fields, the magnetic behavior changes to a superparamagnetic or paramagnetic state. This change is driven by the suppression of the RKKY interaction, caused by the loss of crystal symmetry, SOC effects, and the presence of multiple Dirac cones.